With the technical breakthroughs of manufacturing light emitting diodes (LEDs) in recent years, luminance and luminous efficiency of the LEDs are significantly improved. As a result, the LEDs gradually substitute for traditional light tubes and become new luminous elements. The LEDs are widely applied in illumination fields, such as home lighting devices, automotive lighting devices, hand-held lighting devices, backlight sources of liquid crystal panels, lighting for transportation facilities, lighting for billboards, and so on. To increase the luminance of the LEDs in use, plural LEDs are usually coupled in series to form a series assembly of LEDs.
The LEDs are direct current (DC) loads. In an electronic device in which several series assemblies of LEDs are utilized in parallel, since each LED of the series assemblies of LEDs has different characteristics and different impedance, currents flow through the series assemblies of LEDs are different. As a result, the luminance of the electronic device is not uniform, life spans of the LEDs are significantly decreased, and the electronic device is damaged.
To solve the problem that the currents which flow through the series assemblies of LEDs are not uniform, there are many current balance technologies for improving this deficiency at present. One of these current balance technologies is utilizing independent electrical energy units for respectively driving the series assemblies of LEDs. However, utilizing the independent electrical energy units leads to complicated circuits and high manufacturing cost. Furthermore, the electrical energy units have tolerances, such that a current-sharing effect is poor.
Another technology is utilizing a plurality of common mode choke coils for balancing the currents which flow through the series assemblies of LEDs. However, utilizing the common mode choke coils requires a large quantity of magnetic elements, and thus the manufacturing cost is increased and the size of the electronic device is large. Furthermore, the common mode choke coils generate excitation currents, and thus the current-sharing effect is poor.